"I am also proud to be a mother and a wife," Ruth Kelly told the conference as she bade farewell this morning with an unexpected, unscripted tribute not just to the achievements of the government but of her family too. Once again, the incompatibility of a full-on job with motherhood – elsewhere on Cif Martin Kettle calls it parenthood, but that's aspiration not actuality – is brutally illustrated by a high profile resignation.

It once seemed, briefly, that Westminster had finally found a route to equality. It wouldn't happen overnight, but all women shortlists and public scrutiny of the gender balance in government offered a certainty that it was on its way. Since then the moves have been more backward than forward – and not only numerically. From Nicola Brewer, the new chief executive at the Equalities and Human Rights Commission, warning that legislation risks protecting women out of the job market, to a succession of senior corporate resignations like Anita Smith, chief executive of the insurers Capital Blue Cross, the climate for working mothers is becoming increasingly negative.

It's tough for everyone. But a political career is uniquely demanding. It's not just the hours and days, the late nights and lost weekends, it's doing it all in the unrelenting glare of the public eye. A councillor who lost her seat in May after 10 years remarked to me this morning that politics is the only job where defeat is greeted with cheers and applause.

At Westminster, the career appraisal is not once a year, it's every day. And it's not on the basis of an impartial series of benchmarks, it's prejudice, or political or simply gossip. It's not only women who are deterred by the unconsidered savagery of so much political discourse, of course, but when there so few of them every casualty counts.

Ruth Kelly, with four children all born since she became an MP, a marginal seat and a painful row on the embryology bill looming, had more reasons than most to be considering her position. The suggestion from prime ministerial sources that she was about to be sacked – when she had decided to go anyway – must have incensed her. But her departure robs government of a woman who is not just a daily reminder of what can be done but also of someone who is going through a particular kind of experience. The irony of trying to promote gender equality is that just when you are most motivated to do something about it you are least able to do anything about it. You're bathing the kids.